A number of immunologically active peptides have been isolated from mammalian thymic extracts. Such thymic peptides include the structurally related thymosin alpha-1 thymosin alpha-11 and more recently prothymosin alpha. See in this regard U.S. patent application Ser. No. 546,211, filed Oct. 27, 1983. Prothymosin alpha, identified as a larger peptide containing the thymosin alpha-1 sequence at its amino-terminus, was shown to account for most, if not all, of the immunoreactivity detected with an antibody directed against the amino-terminal sequence of thymosin alpha-1. All the aforesaid thymosin alphas exhibit immuno-potentiating activity and are involved in the regulation, differentiation and function of T-cells.